The electric motor is at the core of most industrial processes, and is usually powered using three-phase AC power that is delivered to the motor's three phase coils through three branches of a motor branch circuit. Industrial motor branch circuit installations can vary greatly in their physical and parametric characteristics, as a function of their application. These include variations in motor size, required power and the physical layout of the circuit itself. Because industrial applications often have very large power requirements, the AC voltage supplied to such circuits can be in the many hundreds of volts, and the current drawn under typical load conditions can be in the tens or even hundreds of amps. Short-circuit currents resulting from faults present at motor start-up can have magnitudes that can threaten human personnel and often damage/destroy components in the circuit that can be costly to replace.
As such, various regulatory organizations have promulgated safety precautions/procedures that must be met for such installations. For example, circuit breakers are required to interrupt the high voltage system power supply from the branch circuit in mid-operation when load currents are detected that exceed the maximum load current of the motor starter by some pre-determined percentage. Because the circuit breakers are relatively slow to react, motor control components are often exposed to currents that exceed the components' maximum rating.
Motor branch circuit components are prone to fatigue and destruction when operated in the presence of such fault conditions. It is common for such faults to exist prior to motor start-up, and can be created inadvertently during maintenance/repair of the circuit. For example, ground faults can be created through the wearing of insulation on the cables, and short-circuits between the phase cables can be established by a metal tool or other materials that are accidentally left across the cable terminations. The motor can experience ground faults through internal coil insulation wear, that can occur over time. These types of faults that are not always readily apparent to personnel during a pre-start-up inspection. Motor control circuit components such as contactors or inverters of motor branch motor circuits can also be compromised internally, leading to ground fault conditions that are not readily discernable from visual inspection.
The technology used in motor branch circuits has remained virtually unchanged for many years. Safety requirements promulgated by various safety organizations do specify the use of various fault detection schemes and components for shutting down an operating motor branch circuit when the presence of faults is detected during normal operation of the motor branch circuit. These regulatory bodies do not, however, specify schemes for prophylactic testing for fault conditions that can detect the existence of such faults before starting the motor circuit. Rather, any testing upon start-up has been heretofore accomplished by simply energizing the branch circuit with system power (typically through closing of circuit breakers) with an expectation of circuit integrity and hoping that motor branch circuit components are not destroyed due to an undiscovered pre-existing fault condition.
If a serious enough fault does exist prior to start-up, components in a motor branch circuit can be weakened or destroyed before the circuit breaker opens the circuit breakers to isolate the motor branch circuit from system power. Personnel can then resolve the fault conditions manually, often without any initial indication of the location or nature of the fault. Moreover, long-developing faults such as ground faults or motor coil faults, which have not yet reached a level of criticality that will cause the safety components to open the circuit upon start-up, will simply not be detected during start-up if they are not yet severe enough to be detected during normal operation. Rather, they may reach an inevitable failure during operation of the installation instead, requiring an emergency shut-down and the potential for costly repairs. Thus, any testing of motor branch circuits for faults largely heretofore has relied on operational detection schemes operating under full system power, which can lead to destruction of components and can create dangerous conditions that must be cleared before operation can resume.